/*
 * $Id: SetCharacterEncodingFilter.java 902 2014-01-23 01:46:10Z tweea@263.net $
 * Copyright(C) 2008 Matrix
 * All right reserved.
 */
package net.matrix.servlet.filter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

/**
 * <p>
 * Example filter that sets the character encoding to be used in parsing the incoming request,
 * either unconditionally or only if the client did not specify a character encoding. Configuration
 * of this filter is based on the following initialization parameters:
 * </p>
 * <ul>
 * <li><strong>encoding</strong> - The character encoding to be configured for this request, either
 * conditionally or unconditionally based on the <code>ignore</code> initialization parameter. This
 * parameter is required, so there is no default.</li>
 * <li><strong>ignore</strong> - If set to "true", any character encoding specified by the client is
 * ignored, and the value returned by the <code>selectEncoding()</code> method is set. If set to
 * "false, <code>selectEncoding()</code> is called <strong>only</strong> if the client has not
 * already specified an encoding. By default, this parameter is set to "true".</li>
 * </ul>
 * <p>
 * Although this filter can be used unchanged, it is also easy to subclass it and make the
 * <code>selectEncoding()</code> method more intelligent about what encoding to choose, based on
 * characteristics of the incoming request (such as the values of the <code>Accept-Language</code>
 * and <code>User-Agent</code> headers, or a value stashed in the current user's session.
 * </p>
 * 
 * @author Craig McClanahan
 */
public class SetCharacterEncodingFilter
	implements Filter {
	/**
	 * The default character encoding to set for requests that pass through this
	 * filter.
	 */
	protected String encoding = null;

	/**
	 * Should a character encoding specified by the client be ignored?
	 */
	protected boolean ignore = true;

	/**
	 * Place this filter into service.
	 * 
	 * @param filterConfig
	 *            The filter configuration object
	 */
	@Override
	public void init(final FilterConfig filterConfig)
		throws ServletException {
		this.encoding = filterConfig.getInitParameter("encoding");
		String value = filterConfig.getInitParameter("ignore");
		if (value == null) {
			this.ignore = true;
		} else if ("true".equalsIgnoreCase(value)) {
			this.ignore = true;
		} else if ("yes".equalsIgnoreCase(value)) {
			this.ignore = true;
		} else {
			this.ignore = false;
		}
	}

	/**
	 * Take this filter out of service.
	 */
	@Override
	public void destroy() {
		this.encoding = null;
	}

	/**
	 * Select and set (if specified) the character encoding to be used to
	 * interpret request parameters for this request.
	 * 
	 * @param request
	 *            The servlet request we are processing
	 * @param response
	 *            The servlet response we are creating
	 * @param chain
	 *            The filter chain we are processing
	 * @exception IOException
	 *                if an input/output error occurs
	 * @exception ServletException
	 *                if a servlet error occurs
	 */
	@Override
	public void doFilter(final ServletRequest request, final ServletResponse response, final FilterChain chain)
		throws IOException, ServletException {
		// Conditionally select and set the character encoding to be used
		if (ignore || (request.getCharacterEncoding() == null)) {
			String selectedEncoding = selectEncoding(request);
			if (selectedEncoding != null) {
				request.setCharacterEncoding(selectedEncoding);
			}
		}

		// Pass control on to the next filter
		chain.doFilter(request, response);
	}

	/**
	 * Select an appropriate character encoding to be used, based on the
	 * characteristics of the current request and/or filter initialization
	 * parameters. If no character encoding should be set, return <code>null</code>.
	 * <p>
	 * The default implementation unconditionally returns the value configured by the
	 * <strong>encoding</strong> initialization parameter for this filter.
	 * 
	 * @param request
	 *            The servlet request we are processing
	 */
	protected String selectEncoding(final ServletRequest request) {
		return this.encoding;
	}
}
